1. Field of the Invention
In the field of high speed impact printing, driving and reversing of the inking ribbon at a controlled velocity and tension is a fundamental requirement. Drum printers are frequently employed when printing speeds of from 200 to 1000 lines per minute are required, in which systems, a ribbon is reversibly transferred between two spools and is interposed between a bank of hammers and a rotating drum of characters. The instantaneous high perpendicular forces produced upon the ribbon by the hammers and the rotating drum during printing cause the ribbon to wear, to gradually lose its ink supply, to track improperly, and ultimately to fail. By maintaining ribbon tension and velocity of travel between the spools as constant as practicable, ribbon life is prolonged as wear and ink use is distributed substantially evenly along the ribbon. Variations in ribbon tension and velocity occur primarily as a result of the changing radii of the ribbon as it winds and unwinds on the spools, which causes constantly changing spool rotational speeds and constantly changing driving motor velocity and torque requirements.
Impact printers operating at speeds of up to 300 to 400 lines per minute employ "tab" ribbons, the typical dimensions of which may be 36 yards by three inches, while higher speed printers typically employ "towel" ribbons which may be 36 yards by 1 foot. The present invention permits the more economical tab ribbons to be used at printing speeds which therefore required the more expensive and cumbersome towel ribbon, and accordingly is described in the preferred embodiment in the context of a tab ribbon system. However, the present invention is also applicable to impact printers of the type in which towel ribbons are used.
2. Description of the Prior Art
Various motor control systems have been used in the prior art to obtain constant tension and speed in transferring and winding material from a take-up reel to a supply reel in which radii of the reels typically vary by a factor of three to one or more. Without compensation for the effect produced by the changing spool radius, gearmotor-torquemotor systems of the prior art result in variation in ribbon speed and ribbon tension of as great as five to one. Prior art attempts to regulate ribbon speed and tension employ extensive servomechanisms and other complex and expensive circuitry and mechanical guides. Effective motor compensation and relatively uniform ribbon tension and speed is provided by the present invention without the complex control servomechanisms and other means of the prior art.
A constant tension-constant speed drive is disclosed by U.S. Pat. No. 3,501,682, in which a two motor system provides constant speed and tension by driving the take-up and supply reel motors in opposite directions to exert an opposing torque on one motor by the counter EMF developed across the field windings of the other motor. Another dual motor control system of the prior art in which winding and unwinding motors are driven in the same direction is disclosed by U.S. Pat. No. 3,079,538, in which the motor velocity and torque are controlled by variation of the motor field winding currents. Yet another plural motor tension and speed control for a magnetic tape drive is disclosed by U.S. Pat. No. 3,295,032, in which motor control is achieved by the use of a servomechanism. Another dual motor control is disclosed by U.S. Pat. No. 3,704,401, in which an error signal is derived from the back EMF of the motors to control a servomechanism which varies the motor speeds. Another dual motor control system of the prior art is disclosed by U.S. Pat. No. 3,715,641, in which the excitation windings of a pair of reel driving motors are oppositely energized to move the reels in opposite directions with the excitation current sum being maintained constant.